Underwater horn loudspeaker



Dec. 26, 1967 r F. MASSA, JR 3,360,771

UNDERWATER HORN LOUDSPEAKE R Filed May 21, 1965 I. 9 2.0 2.\ FREQUENCY \N KC DB. vs. I MICROBAR AT 1 v0 United States Patent Ofiice 3,360,771 Patented Dec. 26, 1967 3,360,771 UNDERWATER HORN LOUDSPEAKER Frank Massa, Jr., Cohasset, Mass., assignor to Massa Division Dynamics Corporation of America, Hingham, Mass.

Filed May 21, 1965, Ser. No. 457,701 9 Claims. (Cl. 340-12) The present invention is concerned generally with underwater loudspeakers and, more particularly, with an improved horn for increasing the efficiency of underwater sound generators. If a vibrating piston is to be used as an underwater sound source and its diameter is small compared to the wavelength of sound corresponding to the frequency of vibration, the acoustic loading on the piston is reduced with a resulting reduction in acoustic power radiation. In order to achieve full acoustic loading on a circular vibrating surface, its diameter should be at least equal to approximately /3 the wavelength of sound being radiated. If the surface is non-circular, then the square root of the area of the vibrating surface should be approximately equal to /3 the wavelength of the radiated sound. For underwater sound generators operating at the lower audio frequencies, in the range of several hundred cycles per second, it is necessary, in order to achieve full acoustic loading, that the dimensions of the vibrating surface be several feet across.

The primary object of this invention is to improve the acoustic loading on the vibrating surface of an underwater sound generator whose surface dimensions are small compared to the wavelength of the radiated sound.

Another object of this invention is to provide a horn structure for use underwater that will serve as an effective acoustic transformer for increasing the acoustic loading on a small vibrating surface, whereby the acoustic power radiated from the vibrating surface is increased.

A still further object of this invention is to provide an underwater horn whose wall density is related to the frequency of sound being transmitted in a novel manner such that full acoustic loading is achieved on the surface of a vibratory structure when it is coupled to radiate sound through the horn opening.

An additional object of this invention is to provide an underwater horn with a non-homogeneous wall structure wherein the horn will be free of self resonances in the frequency range of operation.

These and other objects of the invention will become evident in the following detailed description. The novel features which are characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as advantages thereof, will best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view showing one illustrative embodiment of the present invention, with parts broken away for clarity;

FIGURE 2 is a longitudinal cross-sectional view showing one illustrative embodiment of this invention;

FIGURE 3 is an enlarged detail view illustrating the mounting of an eletcromagnet transducer in a horn; and

FIGURE 4 shows a chart of experimental transmitting response curves measured underwater and illustrates the improved acoustic power output realized by the use of an underwater horn of the present improved design.

It is well known that a vibrating piston whose diameter is small compared to the wavelength of sound being radiated in the medium is poorly loaded by the medium and that the radiation resistance per unit area of the vibrating surface increases as the square of the frequency until the ratio of the piston diameter to the Wavelength of sound being generated becomes equal to approximately /3. If the piston is not of circular shape, the radiation resistance per unit area also increases as the square of the frequency until the square root of the area divided by the wavelength is approximately equal to Further, tapered horns have been used as acoustic transformers to improve the acoustic loading on small vibrating diaphragms for radiating low frequency sound more efiiciently in air. The design of horns for use in the improvement of the acoustic loading for air loudspeakers, has been widely published in the technical literature. See, for example, chapter 8 entitled Horn Type Loudspeakers in the book, Applied Acoustics, 2nd edition, by Olson and Massa published by Blakiston in 1939.

Attempts have heretofore been made to use horns for the improvement of the radiation impedance presented to small vibrating surfaces in an effort to improve the acoustic power output of underwater transducers, especially for transducers designed for low frequency application. Little success has been realized in improving the performance of prior underwater horns. I have found as a result of both theoretical and experimental investigations, that a satisfactory improved underwater horn may be designed and that it will operate in a desired mode if certain critical conditions are fulfilled. A minimum relationship must exist between the weight of the wall section of the horn and the frequency at which the horn is required to operate. Further, I have found that if the specific inertial reactance per sq. cm. area of the wall surface is made comparable to or greater than the specific acoustic radiation resistance of water, which in c.g.s. units is equal to approximately 150,000 ohms/cm. the underwater horn behaves satisfactorily as a true horn structure. The above relationship may be stated by the following equation:

m =mass of wall section of the horn in grams/cm. f=frequency of operation in cycles per second The relationship between the mass of a wall section of the horn, the thickness of the horn wall and the density of the horn wall may be expressed by the equation:

m=td

where:

t=thickness of horn Wall in cm. d=density of horn wall in gm./ cc.

After the above relationship for the minimum weight for the horn wall as a function of frequency of operation has been satisfied, it is advantageous in order to obtain improved performance, that the material used in the construction of the horn be non-homogeneous in character to avoid resonance vibrations of the horn that were found to exist when the horn was constructed as a metallic shell. In a metallic horn, numerous resonant vibrations of the wall are excited during the passage of the sound which will interfere with the normal performance of the horn as an acoustic transformer.

One of the non-homogeneous materials which may be advantageously used for the horn construction is an aggregate of concrete and scrap metal having an average density in the general neighborhood of lbs. to 300 lbs. per cu. ft. As is apparent, the density of the wall section of the horn is greater than the density of water.

This range of density of the metal reinforced concrete is not critical to the operation of the improved horn, provided that the thickness of the wall is adjusted to satisfy the relationship indicated in Equation 1 for the frequency of operation.

Further improvement in operation of the horn structure results from making the inner flared surface of the horn smooth. A suitable hard waterproof bonding material, such as a hard epoxy varnish, may be used to fill any pores which may be present in the concrete surface. Preferably, the passageway of the horn is flared or tapered so that the area of the opening increases by constant percentage increments for equal distances along the axis and that the large opening of the horn has an area which is greater than A /ZS where is the wavelength corresponding to the frequency being radiated.

Referring to FIGURES 1, 2 and 3, the numeral illustrates a transducer which is adapted to oscillate along a line parallel to the axis of the horn 11. The transducer may be of any conventional type that can operate underwater. One suitable type of structure is an electromagnetic transducer comprising an inertial inner mass which is flexibly suspended from and drives a rigid outer shell structure by virtue of oscillating magnetic forces generated between the inner and outer mass portions of the transducer. Further description of a satisfactory transducer for operating in deep water and which may be used as a transducer in combination with the underwater horn described in this invention is given in the copending application Ser. No. 334,203, now Patent No. 3,308,423, filed on Dec. 30, 1963, by Frank Massa, Jr., and entitled, Electroacoustic Transducer.

The present invention is not concerned with the transducer design which is indicated generally by the numeral 10. The drawing illustrates the association of the vibrating surface of the transducer with the small opening of the horn 11. The transducer 10 is mounted so that it may be freely slid axially into the smooth tubular liner 12 which is part of the horn structure. An annular resilient gasket 13, made from rubber or like material, separates the radiating face 14 of the transducer from the rigid surface of the horn. The transducer 10 is held in place in horn 11 by means including resilient pads 15 that are held in place against surface 16 of transducer 10 by the annular clamping strips 18, which are fastened in position by the bolts 20.

The horn 11 is preferably made of a non-resonant aggregate which may advantageously consist of cast concrete containing a filler of random-shaped pieces or bits of scrap metal. The bits of metal serve to increase the density of the horn structure and also serve to produce a non-homogeneous mass which will eliminate any predominant self-resonances that could occur in the bellshaped horn if the wall were fabricated of homogeneous material. The existance of any horn resonances within the desired frequency of operation could cause secondary sound radiation from the horn surfaces, which would interfere with the direct radiation of sound through the horn opening.

The surface 24 defining the passage through the horn tapers or flares from the small opening 26 to the large opening 28. As aforenoted, the cross-sectional area of the passageway increases exponentially from the small opening to the large opening, that is, the area increases by constant percentage increments for equal distances along the axis of the horn 11.

FIGURE 4 shows a graph of experimental data taken underwater with a transducer having a piston approximately five inches in diameter which was designed for operation in the 2 kilocycle region. Curve A shows the measured sound pressure level at 1 yard distance from the transducer without the horn. The amplitude of transducer face at the highest point on the response curve was measured as .00015 inch peak to peak.

Curve B shows the measured sound pressure level at 1 yard distance from the transducer and horn assembly mounted as illustrated in FIGURE 1. The horn used to obtain the data for curve B was cast with a scrap iron and concrete mixture having a density of approximately 200 lbs/cu. ft. and the wall thickness was adjusted to satisfy the approximate minimum requirement of Equation 1, namely, the mass of the wall was about 13 grams for each sq. cm. of area of the wall surface.

Curve C shows the measured response with a horn having the same inner shape but the wall thickness was doubled such that the mass per unit area of the wall surface was approximately 26 gms./crn. or twice the minimum value indicated in Equation 1.

For each of the three conditions represented by curves A, B, and C, the amplitude of the transducer face was held constant at .00015 inch peak to peak at the frequency of maximum response. The large indicated increase in acoustic power radiation between curve A and curve B illustrates very strikingly the improved operation of the underwater horn designed according to the teachings of this invention. The relative improvement of curve C over curve B requires a horn of approximately double the weight and the desirability of realizing such an additional improvement in performance will depend on the importance of realizing the increased efliciency of operation versus the economic factors to be considered in the manufacture and handling of the heavier horn.

Although the basic principles of this invention have been described in connection with presently preferred embodiments, it will be obvious to those skilled in the art that numerous departures may be made from the specific details shown, and therefore, it is contemplated that the invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

What is claimed and desired to be protected by Letters Patent of the United States is:

1. In combination in an underwater horn loudspeaker, a vibratory structure having a vibratory surface capable of performing oscillatory displacements, said vibratory surface characterized in that the square root of its area is less than /3 the wavelength of sound being radiated in the water by the vibrating surface, a horn structure made from a rigid non-homogeneous aggregate having a density greater than the density of water, said horn structure having a tapered passage along its length from a small opening at one end to a large opening at the other end, the small end of the horn structure being in close proximity to the vibratory surface of said vibratory structure for receiving the compressional waves created by said vibratory surface directly in the small opening of said horn structure.

2, The invention set forth in claim 1 wherein the crosssectional area of the tapered horn passage increases by constant percentage increments for equal linear distances along the axis of the horn structure from the small opening at one end to the large opening at the other end.

3. The invention set forth in claim 1 wherein the large opening of the horn structure has an area which is greater than A /25, where A is the wavelength of the zound being generated in the water by the vibratory surace.

4. In combination in an underwater horn loudspeaker, a vibratory structure having a vibratory surface capable of performing oscillatory displacements, said vibratory surface characterized in that the square root of its area is less than A the wavelength of sound being radiated in the water by the vibrating surface, a horn structure made from a rigid non-homogeneous aggregate having a density greater than the density of water, said horn structure having a tapered passage along its length from a small opening at one end to a large opening at the other end, the small end of the horn structure being in close proximity to the vibratory surface of said vibratory structure for receiving the compressional Waves created by said vibratory surface directly in the small opening of said horn structure, the cross-sectional area of the tapered horn passage increasing by constant percentage increments for equal linear distances along the axis of the horn structure from the small opening to the large opening, and the large opening of the horn having an area which is greater than V/ 25, where A is the wavelength of the sound being generated in the water by the vibratory surface.

5. In combination in an underwater horn loudspeaker, a vibratory structure having a vibratory surface capable of performing oscillatory displacements, said vibratory surface characterized in that the square root of its area is less than /3 the wavelength of sound being radiated in the water by the vibrating surface, a horn structure having a flared passage along its length from a small opening at one end to a large opening at the other end, the small end of the horn structure being in close proximity to the vibratory surface of said vibratory structure for receiving the compressional waves created by said vibratory surface directly in the small opening of said horn structure, the weight of the walls of the horn structure being greater than 25,000/f grams per sq. cm. of wall area, where f is the frequency of vibration of the vibratory surface in cycles per second.

6. The invention set forth in claim 5' wherein the horn structure is made from structural material comprising a rigid non-homogeneous aggregate having a density greater than the density of water.

7. The invention set forth in claim 6 wherein said structural material is concrete.

8. The invention set forth in claim 6 wherein said structural material is concrete reinforced with pieces of random-shaped metal.

9. The invention set forth in claim 5 wherein the surface defining said flared passage is lined with a hard waterproof bonding material to smooth the surface defining the flared passage and improve the operation of the horn structure.

References Cited UNITED STATES PATENTS 1,126,095 1/1915 Schiessler. 1,623,561 4/ 1927 Slepian et al. 2,338,262 1/1944 Sahnon Isl-27 2,447,333 8/1948 Hayes. 2,617,874 11/1952 Lewis.

RODNEY D. BENNETT, Primary Examiner.

BENJAMIN A. BORCHELT, Examiner.

M. F. HUBLER, Assistant Exwminer. 

1. IN COMBINATION IN AN UNDERWATER HORN LOUDSPEAKER, A VIBRATORY STRUCTURE HAVING A VIBRATORY SURFACE CAPABLE OF PERFORMING OSCILLATORY DISPLACEMENTS, SAID VIBRATORY SURFACE CHARACTERIZED IN THAT THE SQUARE ROOT OF ITS AREA IS LESS THAN 1/3 THE WAVELENGTH OF SOUND BEING RADIATED IN THE WATER BY THE VIBRATING SURFACE, A HORN STRUCTURE MADE FROM A RIGID NON-HOMOGENEOUS AGGREGATE HAVING A DENSITY GREATER THAN THE DENSITY OF WATER, SAID HORN STRUCTURE HAVING A TAPERED PASSAGE ALONG ITS LENGTH FROM A SMALL OPENING AT ONE END TO A LARGE OPENING AT THE OTHER END, THE SMALL END OF THE HORN STRUCTURE BEING IN CLOSE PROXIMITY TO THE VIBRATORY SURFACE OF SAID VIBRATORY STRUCTURE FOR RECEIVING THE COMPRESSIONAL WAVES CREATED BY SAID VIBRATORY SURFACE DIRECTLY IN THE SMALL OPENING OF SAID HORN STRUCTURE. 